Although Henry Hudson's name is now entirely associated with north America, his aim in the first three of his four voyages is to find a northeast passage rather than one to the northwest.

He is commissioned first by the English merchants of the Muscovy Company, in 1607 and again in 1608, to push further the exploration undertaken by Willem Barents - in the hope of finding a route round Novaya Zemlya which will lead to the east. On each occasion he fails to go further than his predecessor. In April 1609 he departs again, this time sailing on behalf of the Dutch and with a broader brief. He is to seek a passage either northeast or northwest.

Once again Hudson heads northeast. But when nearly surrounded by ice, near Novaya Zemlya, he is confronted by a mutinous crew. He pacifies them by agreeing to pursue the other part of his commission, involving less northerly waters.

His ship, the Half Moon, turns west across the Atlantic and reaches America in the vicinity of Virginia. Hudson searches northwards up the coast, and turns into the great inlet of New York Bay. Exploring the river now known by his name, he soon realizes that it is not the strait he is looking for. In October 1609 he turns for home.

The English authorities now forbid Hudson to serve the Dutch, so his last journey is again for a consortium of English merchants. He is to explore north of the St Lawrence, already proved by Cartier to be a river rather than a strait. He leaves England, in the Discovery, in April 1610.

During June the Discovery sails into a broad strait, and in early August Hudson arrives in a massive bay. Both strait and bay are now known by his name. He turns southwards in Hudson Bay and reaches its most southerly part (James Bay) in late October.

Hudson makes his winter quarters in James Bay. He and his crew survive the dark freezing months, but not without discontents. There is a mutiny when the return journey begins. On 22 June 1611 Hudson and eight others, including his son, are set adrift in an open boat. Nothing more is heard of them. The chief mutineer dies on the journey home. The pathetic remnant of the crew, reaching England in September 1611, are treated leniently.

The interest generated by Hudson's expedition hastens English involvement in the fur trade of northern Canada. He has not been the first European to explore either the strait or the bay which have made him famous. But he has been the most thorough. And his adventure has been the most dramatic.

Terra Australis: 16th-18th century

From the early 16th century European merchants are sailing the seas of southeast Asia. Often they make unexpected landfall, raising hopes of unknown territories rich in gold, silver or spice. The discovery of the Solomon Islands by a Spanish vessel in 1568 prompts interest in a so-called Terra Australis Incognita ('unknown southern land'). Part of the brief given to Francis Drake, when he sets off in 1577 to sail across the Pacific, is that he should search for this supposed land of treasure (see Drake's voyage).

Interest is maintained in the early 17th century when Dutch ships, sailing to and from the Moluccas, sight stretches of the western Australian coast. Are these places perhaps connected to the southern land?

The governor general of the Dutch East Indies, Antonio van Diemen, decides to investigate. He chooses for the purpose an experienced navigator, Abel Tasman, who is instructed to sail far south in the Indian Ocean and then to strike east, hoping to discover whether there is an open passage to South America. In the process he may also perhaps discover Terra Australis.

Tasman leaves Batavia in August 1642. He sails to Mauritius before continuing south and then east. He first makes landfall in November. He calls the place Van Diemen's Land, after the governor who has appointed him. Not until 1856 is the island renamed Tasmania, in honour of its discoverer.

Keeping to the southern coast of this large island, Tasman continues eastwards. In December he reaches New Zealand. Sailing northeast along the coast of both South and North Island, he concludes that this must be the northwest corner of Terra Australis. Tasman discovers Tonga in January 1643, and the Fiji islands in February. He then continues northwest, passing north of New Guinea and returning to Batavia in June.

Remarkably, in his ten-month voyage, Tasman has sailed all the way round the real Terra Australis without noticing it. It will be another century before the continent of Australia is properly discovered and charted.

Ohio and Mississippi: 1669-1682

The great central valley of north America, watered by the Ohio, Mississippi and Missouri rivers, is first visited by Europeans during the late 1660s and 1670s. This development is the direct result of the growth of the colony of New France during the 1660s. As the French explore through and around the Great Lakes, they begin also to move down the rivers running south from this region.

The nearest large river to the eastern lakes, and the first to receive attention, is the Ohio. Robert de La Salle explores the Ohio valley during 1669, in a journey which provides the basis for the later French claim to this area.

Four years later a much more dramatic expedition is undertaken by a trader, Louis Jolliet, and a Jesuit priest, Jacques Marquette (founder in 1668 of the mission at Sault Sainte Marie). With five companions, in 1673, they make their way round Lake Michigan in two birch bark canoes. From Green Bay they paddle up the Fox river, before carrying their canoes overland to the Wisconsin and thus on to the Mississippi.

They travel down the Mississippi as far as its junction with the Arkansas river, by which time they are convinced that it must flow into the Gulf of Mexico rather than the Pacific. With this information they make their way back to Lake Michigan.

Inspired by their example, La Salle becomes determined to reach the mouth of the Mississippi. After two false starts, several disasters and a long struggle for funds, he finally achieves the task in 1682. At the mouth of the great river he claims possession for France of the entire region drained by the Mississippi and its many tributaries, naming it Louisiana - in honour of his monarch, Louis XIV.

It is some time before the southern region becomes a desirable colony, though there is a brief flurry of excitement with John Law's Mississippi Scheme of 1717 and the founding of New Orleans in 1718. But the Ohio valley is a region of great significance in the 18th century, being hotly disputed between the French and the British.

Bering's Voyages: 1728-1741

The Danish explorer Vitus Bering is an officer in the Russian navy when he is appointed by Peter the Great, in 1724, to lead an expedition to discover whether Siberia is joined to the continent of America.

Bering builds a ship at the mouth of the Kamchatka river, in eastern Siberia, and sails north in July 1728 through what is now the Bering Sea. He makes his way up through the Bering Strait into the Arctic Circle, seeing and naming the Diomede Islands in the middle of the channel. On this occasion he fails to sight the coast of Alaska.

On a second voyage, in 1741, Bering reaches Alaska and explores part of its southern coast. On the way back to the Kamchatka peninsula his ship is wrecked on one of the Komandorski islands. Bering dies that winter, but survivors of his expedition get back to Russia with furs purchased in Alaska.

The lure of fur brings merchants eastwards and begins Russia's link with Alaska - formalized at the end of the century in the Russian-American Company.

Discovery of the Pacific islands: 18th century

During the 18th century the maritime powers of northwest Europe make an increasingly coherent effort to discover which remote islands may be lurking in the middle of the vast Pacific. Dutch, French and English vessels undertake voyages of discovery, gradually filling in the map.

Islands are regularly discovered during the century. Among the better known, Easter Island is reached by the Dutch in 1722, Tahiti by the English in 1767, the New Hebrides by the French in 1768, and New Caledonia and Hawaii by the English (in the person of Captain Cook) in 1774 and 1778.

Three voyages of Captain Cook: 1768-1779

The voyages of James Cook are the first examples of exploration undertaken on scientific principles. His first expedition, sailing in the Endeavour from Plymouth in 1768, has a scientific task as its central mission. It is known to the astronomers of the day that in June 1769 the planet Venus will pass directly between the earth and the sun. An international effort is made to time the precise details of this transit, as seen from different parts of the world, in the hope of calculating the earth's distance from the sun.

Cook first mission is to sail to Tahiti, set up a telescope for this purpose and take the necessary readings.

Cook's second purpose is exploration. He is to continue the search for the supposed southern land, Terra Australis, and he is to chart the coast of the known territory of New Zealand. He has among his passengers scientists of another discipline. The botanists Joseph Banks and his Swedish colleague Daniel Solander are eager to collect specimens of Pacific flora.

Cook observes the transit of Venus in the summer of 1769 and then spends the next eighteen months charting the entire coast of New Zealand's two main islands and the east coast of Australia. The Endeavour is back in Britain in July 1771.

The original astronomical purpose proves the least significant part of the voyage (the data proves inadequate for the intended purpose). But Cook's charting of these important coast lines is carried out to a scientific standard previously unattempted. As the first Europeans to visit Australia's congenial eastern coast, the reports of Cook and his distinguished passengers are instrumental in encouraging the notion of forming British settlements. And the botanical specimens of Banks and Solander prove of immense value.

One issue not resolved is whether there is an unknown southern continent south of New Zealand. Cook now proposes another voyage to more southerly latitudes.

Cook sails from England in 1772 (now in the Resolution) and spends the three antarctic summers of 1772, 1773 and 1774 in a complete circumnavigation of the ice mass of the south pole - proving finally that there is no unknown habitable continent in the south (though Cook suspects, rightly, that there may be land under the ice).

Back in England in 1775, Cook reveals another scientific aspect to his explorations. His crew have remained surprisingly healthy in these long voyages, avoiding the sailor's debilitating disease of scurvy. Cook publishes a paper on his method for avoiding this condition. His men are given a regular ration of lemon juice.

Cook has discovered the importance of vitamin C, long before the substance itself is identified. The navy adopts his method, later substituting lime juice for lemon (causing British sailors in foreign ports to be known as 'limeys').

Cook's aim on his third voyage (again in the Resolution, from 1776) is to explore the Pacific coast of north America. He sails through the Bering Strait as far as the pack ice of the north pole. On his outward journey he discovers the Hawaiian group of islands, and here - wintering in Hawaii itself - he is killed in a skirmish with natives. He has spent all but two of the past ten years at sea, making an unprecedented contribution to knowledge of the Antarctic seas and the Pacific.

Northwest Canada:1789-1793

The four years from 1789 bring much new knowledge about northwest Canada, particularly from two great journeys carried out by a Scottish fur trader, Alexander Mackenzie. By the 18th century Canada has already been well explored, mainly by fur traders, to some considerable distance west of the Great Lakes and Hudson Bay. Mackenzie has been living for some years at the extremity of the known region, with his base at the Indian trading post of Fort Chipewyan on Lake Athabasca.

A great river flows northwest out of Lake Athabasca. In early June 1789 Mackenzie sets off with a small party in birch-bark canoes to discover where it leads.

A week brings them to the Great Slave Lake, which they find covered in ice too solid for their canoes but too fragile to walk on. They carry the canoes round its edge until they come to a river (now the Mackenzie) emerging from its western extremity. They follow this to its outlet, at Mackenzie Bay, into the Beaufort Sea - a part of the Arctic Ocean beyond the as yet undiscovered northwest passage. Mackenzie and his party are back in Fort Chipewyan in mid-September, having canoed about 3000 miles in not much more than three months.

With an unsated appetite for adventure, Mackenzie sets off again from Fort Chipewyan in 1792 on an even more ambitious undertaking - to reach the Pacific.

There has as yet been no recorded crossing of the continent north of Mexico, and it is unlikely that any unknown American Indian was ever tempted by the task which Mackenzie undertakes in July 1792. From Fort Chipewyan he travels along and between a succession of rivers, and then through the Canadian Rockies, to reach the coast at the mouth of the Bella Coola river in June 1793.

Mackenzie is unaware of it, but another explorer is in the region at exactly this same moment. Mackenzie reaches the sea about 100 miles north of Vancouver Island, named after George Vancouver who is spending two years surveying the coast from California to Alaska. In 1792 he becomes the first captain to sail round Vancouver Island.

Vancouver has begun his career as an able seaman on Cook's second voyage (1772-5) and has graduated to midshipman for the third voyage (1776-80). Now, leaving Britain in 1791 in command of his own expedition to the Pacific, he takes pride in applying Cook's high standards of surveying and cartography. Indeed on occasion there is a humorous delight in upstaging the master. On the chart for Dusky Bay in New Zealand where Cook, lacking time to investigate, has written 'Nobody knows what', Vancouver now fills in the coastline and the words 'Somebody knows what'.

Thanks to Mackenzie and Vancouver, by 1793 somebody knows a great deal more than before about northwest Canada.

The challenge of Africa: from1788

The ability of European ships to sail anywhere on the oceans of the world - culminating in the great voyages of Captain Cook in 1768-79 - means that by the end of the 18th century the coast lines of the continents are familiar. So, from many centuries of to and fro, are the interior regions of Europe and Asia.

The interiors of the other three continents remain largely a mystery. North America will soon have heroic tales of exploration (particularly that of Lewis and Clark in 1804-6) and Australia's fearsome outback will claim tragic victims (such as Burke and Wills). But it is the ancient continent of Africa which now most fires the imagination of explorers, particularly in Britain.

The mouths of Africa's great rivers - the Nile, the Niger, the Congo, the Zambezi - are familiar to European traders. And there are reliable reports of great stretches of inland river (particularly the Niger, linked in history with several important African kingdoms). But no one has any idea how it all joins up. Where do the inland rivers reach the sea? Where do the estuary waters come from? These questions tantalize explorers from the late 18th century to the heady days of Livingstone, Burton and Speke, Baker and Stanley.

In 1788 an African Association is founded in London. Trade is one of its aims. Another is exploration, and specifically the discovery of the course of the Niger.

Mungo Park and the Niger: 1795-1806

Mungo Park, by trade a ship's surgeon, hears of the African Association's interest in the Niger and in 1794 offers his services as an explorer. From June 1795 he travels 200 miles up the river Gambia and then sets off overland into unknown territory. After a highly eventful journey he reaches the Niger in July 1796, but is only able to travel a short way down the river before he has to turn for home - exhausted and insolvent. He makes his way safely back to Britain, where his account of his adventures (Travels in the Interior of Africa, 1799) is an immediate success.

In 1803 the British government decides to sponsor another expedition to the Niger. Park is invited to lead it.

As an official undertaking, the new expedition is large and relatively well equipped. But this time Africa shows its fearsome credentials rather more emphatically. Forty men set off up the Gambia in 1805. By the time Park reaches the Niger his companions have been reduced to eight; fever and dysentery have killed the others.

In November 1805 the party of nine sets off in canoes from Ségou, to make their way down unknown stretches of the river. No more is heard of them, until vague reports of disaster filter back to British settlements on the Gambia. Eventually it is discovered that the explorers travelled 1000 miles down the river to the region of Bussa, where they died after an attack by natives.